The present invention relates to a system making it possible to obtain a selective reaction in photochemical processes on the basis of laser beams incorporating means for distributing said beams.
The invention applies to photochemical processes requiring the combined action of several light emissions of different wavelengths in order to obtain a selective reaction, such as an isotopic separation or a photoisomerization.
Isotopic separation can e.g. be used for eliminating an isotope which is incompatible with the industrial use of a product, such as in the case of purifying metals or for selecting a useful isotope, e.g. carbon or uranium.
By the rearrangement of the atoms of a molecule previously excited by light emissions, the photoisomerization of molecules makes it possible to obtain a molecule having different spectroscopic and chemical properties.
To obtain the sought selective reaction, it is possible to proceed in known manner in two stages. The first stage consists of selectively exciting an isotopic or chemical species on the basis of one or more laser radiations. The second stage consists of causing the transformation of the previously excited species by a final laser radiation having an adequate energy.
The selective excitation of the species, i.e. the molecule or atom, takes place in known manner by successive passages of the molecule or atom in question to levels having an ever higher energy by absorption of photons, each photon coming from a pulse laser with a particular wavelength.
In certain cases, selective excitation can be obtained by absorption of a single photon and therefore by the passage of the molecule or atom in question to a single energy level.
The excited species is transformed by irradiating it with a laser beam of a wavelength such that it clears the level corresponding to said transformation. This leads either to the formation of a new molecule or to the ionization of a molecule or atom. In this way, it is possible to distinguish the species formed from other species and separate them.
FIG. 1 shows an example of transitions at several levels in U.sup.235 making it possible to bring about its ionization. Thus, to separate the isotope U.sup.235 from uranium vapour, use is made of a selective excitation beam S.sub.1 constituted by two beams S'.sub.1 and S".sub.1 of respective wavelengths .lambda.'.sub.1 and .lambda.".sub.1, which bring the atoms of isotope U.sup.235 to two successive levels 1, 3. A final beam S.sub.2 of wavelength .lambda..sub.2 brings the excited atoms of U.sup.235 into an ionization state 5. The ionization energy of isotope U.sup.235 is equal to 6.12 eV, so that each of the wavelengths .lambda.'.sub.1, .lambda.".sub.1, .lambda..sub.2 is approximately 600 nm.
In order to optimize the isotopic separation of U.sup.235, use can be made of a fourth wavelength .lambda..sub.1 " associated with a beam S.sub.1 "' in order to bring the atoms already at an intermediate energy level 7, occupied by a thermal process, to level 1, so as to be ionized following successive irradiations at wavelengths .lambda.".sub.1 and .lambda..sub.2.
Throughout the remainder of the text, S.sub.1 will be used for the beam permitting the selective excitation of the species in question, whereby S.sub.1 can contain a single wavelength .lambda..sub.1 or can result from a superimposing of beams S'.sub.1, S".sub.1, . . . S.sub.1.sup.(n) of wavelengths .lambda.'.sub.1, .lambda.".sub.1, . . . .lambda..sub.1.sup.(n), with n being an integer equal to at least 1 and S.sub.2 is the beam of wavelength .lambda..sub.2 permitting the ionization or photodissociation of the previously excited species.
The different wavelengths are obtained in known manner from dye lasers, (e.g. rhodamine lasers) excited by other lasers, which can be copper vapour lasers. This gives pulse-type light emissions of a few dozen ns and a repetition frequency of a few kHz.
In known manner, beams S.sub.1 and S.sub.2 are transmitted in the same propagation direction into an enclosure containing the substance from which a chemical or isotopic species is to be extracted and which is in the form of a vapour flow. The effective absorption sections of the transitions corresponding to the selective excitation and transformation of the species in question can differ. The effective absorption sections of the transitions corresponding to the selective excitation can be 10 to 100 times greater than that corresponding to the transformation. Moreover, to retain a good selectivity, an excessive power of beam S.sub.1 must not be used, because this would lead to a loss of selectivity resulting e.g. from broadening through saturation, or to transitions with several photons. Following interaction of the beams S.sub.1 and S.sub.2 with the species in question, beam S.sub.1 is very attenuated compared with beam S.sub.2. Thus, the simultaneous presence of these two beams cannot be maintained throughout their passage in the enclosure. As a result of this interaction beam S.sub.2 is not very well used, its energy being wasted in the final part of the path where beam S.sub.1 is highly attenuated.
Thus, the prior art means do not make it possible to optimize the use of these beams.